Various neurotransmitters and their functions

  • Neurotransmitters are chemical agents synthesized by nerve cells.
  • Are stored in secretory vesicles in axon terminals.
  •  Released when a nerve impulse reaches the end bulb.
  • Also defined as the chemicals which allow the transmission of signals from one neuron to the next across synapses.
  •  Produce an excitatory or inhibitory response in the post synaptic membrane.
  • Released from synaptic vesicles in synapses into the synaptic cleft.
  • In synaptic cleft, they are received by neurotransmitter receptors on the target cells.
  • For example: acetylcholine evokes an excitatory response at the motor end plate in a skeletal muscle cell. whereas it evokes an inhibitory response at its synapse with a cardiac muscle cell.
  • Many possible neurotransmitters are being investigated.
  • There are four main criteria for identifying neurotransmitters:
  1. The chemical must be synthesized in the neuron or otherwise be present in it.
  2. When the neuron is active, the chemical must be released and produce a response in some target.
  3. The same response must be obtained when the chemical is experimentally placed on the target.
  4. A mechanism must exist for removing the chemical from its site of activation after its work is done.
  • About 50 are present in our nervous system and there may be over 100.
  • Known neurotransmitters are classified into four groups. They are:

1) Acetylcholine

  • It is the main neurotransmitter released by neurons of the peripheral nervous system.
  • Only a restricted number of neurons in the central nervous system release acetylcholine.
  • The acetylcholine released by lower motor neurons stimulates contraction of skeletal muscle.
  • In contrast, the acetylcholine released by the postganglionic neurons of the peripheral nervous system inhibits contraction of cardiac muscle.

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2) The amino acids neurotransmitters

  • Amino acid neurotransmitters release (exocytosis) is dependent upon calcium Ca2+and is a presynaptic response.
  • There are inhibitory amino acids (IAA) or excitatory amino acids (EAA).
  • Some of them include gamma-aminobutyric acid (GABA), glycine, aspartic acid, and glutamic acid.
  • Although GABA is not an amino acid, it is classified with the amino acids because it is derived from glutamic acid.
  • It is the major inhibitory transmitter of the small local circuit neurons in such structures as the cerebral cortex, cerebellum, and upper brainstem.
  • Glycine, the simplest amino acid, is the major inhibitory transmitter of local circuit neurons in the lower brain stem and spinal cord.
  • It selectively inhibits certain neurons to refine and focus brain activity into meaningful patterns by eliminating and suppressing nonessential activity.
  • Glutamic acid and aspartic acid are excitatory transmitters in the central nervous system.

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3) Monoamines

  • Monoamine neurotransmitters are neurotransmitters and neuromodulators.
  • Contain one amino group connected to an aromatic ring.
  • connection occurs by a two-carbon chain (such as -CH2-CH2-).
  • Include catecholamines, such as norepinephrine and dopamine, and indoleamines, such as serotonin.
  • Nor-ephinephrine (noradrenaline) is located in neurons with cell bodies in the brain stem and in post-ganglionic neurons of the sympathetic nervous system.
  • Norepinephrine releasing fibers are distributed widely in structures such as the cerebral cortex, cerebellum and spinal cord.
  • Fibers that terminate in the cerebral cortex are involved with various levels of consciousness.
  • Pep pills containing the drug amphetamine increase the level of norepinephrine in the brain by blocking its reuptake in the neuron and inhibiting the action of enzyme monoamine oxidase (MAO).
  • The euphoria and hallucinations produced by amphetamines are thought to result from the inhibition of MAO.
  • An amphetamine psychosis which is quite similar to schizophrenia can result from the habitual use of amphetamines.
  • Chlorpromazine is used in the treatment of schizophrenia and similar disturbances.
  • It acts by blocking the reuptake of norepinephrine.
  • Dopamine is neurotransmitter found in the area of brain where certain neural circuits are involved with voluntary motor integration.
  • Deficits of dopamine involving the basal ganglia are associated with Parkinson’s disease.
  • Serotonin is associated with various mood swings, including depression, elation, insomnia, and hallucinations.

4) Neuropeptides

  • Chains of neuroactive aminoacids.
  • Found in the central nervous system (CNS).
  • Somatostatin, the endorphins and the enkephalins are examples.
  • Release of growth hormone from the pituitary gland is acted upon by somatostatin.
  • Inhibitory function is shown by somatostatin to growth hormone.
  • Apparently, it acts by altering the excitability of the post synaptic membrane of target cells.
  • Endorphins (morphine like substances) and enkephalins (in the head substances) are naturally occurring peptides.
  • Found in several regions of the central nervous system.
  • Function at post synaptic receptor sites in the pain pathways.
  • Suppress synaptic activity leading to pain sensation.
  • These opiates are not used therapeutically because their effectiveness is relatively short-lived and because they are addictive.
  • Many neuropeptides are classified as neuromodulators.
  • They are the chemical agents that are capable of altering the responsiveness of neurons to a neurotransmitter.
  • Neuropeptides, histamines, prostaglandins, and the hormones cortisol and estrogen are thought to have neuromodulator functions.

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Various neurotransmitters and their functions